This invention relates to a resin coated sand for the fabrication of molds and cores for use in sand mold casting and a process of preparing the same, and more particularly to a novel type of binder for the preparation of the coated sand.
In sand mold casting of aluminum alloys, molds and cores are usually made of resin coated sand, i.e. casting sand coated with a phenol resin. This type of coated sand has been used also in casting of cast irons. In the case of cast irons, molds and cores of resin coats sand can easily be disintegrated after solidication of the poured molten iron since a relatively high pouring temperature such as 1300.degree.-1400.degree. C. causes the resin to decompose and lose its bonding ability. However, the situation is different in casting of aluminum alloys. Due to the adoption of a relatively low pouring temperature such as 650.degree.-750.degree. C., molds and cores of resin coated sand retain their toughness even at the stage of shake-out and accordingly resist shocks and vibrations for shake-out. This is particularly significant for cores. In a relatively thick portion of a core, curing of the phenol resin proceeds during solidification of the poured molten metal because the interior of the core in such a portion does not undergo an efficient cooling. Even in a relatively thin portion of the core, the phenol resin undergoes partial carbonization mainly in its benzene rings because the core surrounded by the molten metal undergoes heating in an anoxic state. As a result, the sand particles adhere strongly to each other or to the aluminum alloy casting.
Because of difficulty in disintegrating molds and cores of resin coated sand, when casting of an aluminum alloy is performed by the use of complicated shaped cores it is usual to facilitate disintegration of the molds and cores by a shake-out machine by preliminarily baking mold assemblies containing castings for a period of time as long as 4-10 hours at 400.degree.-500.degree. C. This is of course unfavorable to the efficiency and cost of the casting process.
It may be concluded that the use of phenol resin as a binder for casting sand is not so desirable in general and, in the case of casting of aluminum alloys, is certainly undesirable.
A resin-base binder for the preparation of a coated casting sand should have such physical and chemical properties that molds and cores made of the coated sand are strong enough to serve for the purpose but, nevertheless, can easily be disintegrated after solidification of the poured molten metal. Furthermore, such a binder is desired to be solid and practically free of tackiness at room temperature so that the individual particles of the coated sand may not adhere to each other unless pressure or heat is applied thereto, but at a temperature slightly above its softening temperature becomes sufficiently low in its viscosity so that sand particles may be well wetted with the fluidized binder. For casting sand, however, none of the currently available or hitherto proposed binders simultaneously satisfies alloy these desires.